The present invention relates to a superconducting gradiometer coil system for an apparatus for the multi-channel measurement of weak nonstationary magnetic fields in a field intensity range of less than 10.sup.-10 T, and especially less than 10.sup.-12 T. This apparatus includes in each channel, in addition to at least one detection coil and at least one compensation coil, a superconducting quantum interference device (SQUID) as well as corresponding superconducting connecting means connecting the components, and further comprises electrical apparatus for the evaluation, processing and displaying of the information obtained at the quantum interference elements.
The use of superconducting quantum interference devices which are generally called "SQUIDs" (short for "Superconducting Quantum Interference Devices") is generally known for the measurement of very weak magnetic fields. See, e.g., J. Phy. E.: Sci. Instrum.", vol. 13, 1980, pages 801 to 813 and "IEEE Transactions on Electron Devices", vol. ED-27, no. 10, October 1980, pages 1896 to 1908. A preferred field of application for these devices is medical technology, particularly magnetocardiography and magnetoencephalography, where magnetic heart and brain waves with field intensities on the order of magnitude of 50 pT and 0.1 pT respectively occur. See, e.g., "Biomagnetism-Proceedings of the Third International Workshop on Biomagnetism, Berlin 1980", Berlin/New York, 1981, pages 3 to 31, and "Review of Scientific Instruments", vol. 53, no. 12, December 1982, pages 1815 to 1845.
A device for measuring such biomagnetic fields contains substantially the following components:
1. A SQUID as the field sensor proper with a so-called gradiometer;
2. A flux transformer in the form of a coil arrangement for coupling-in the field to be examined from the gradiometer to the SQUID;
3. Electronic equipment for signal acquisition and processing;
4. Shielding for the earth's magnetic field and external interference fields; and
5. A cryosystem for ensuring superconduction of the sensor and the gradiometer.
The design and operation of such devices with a single channel are known. In these devices, the magnetic field to be detected which is up to 6 orders of magnitude smaller than external interference fields, is inductively coupled, generally via a coil arrangement of superconducting wire, to the circuit with a Josephson contact formed by a radio-frequency (R-F)SQUID. Coil systems called gradiometers of first or higher order are formed by the combination of a sensor coil, also called a detection coil, with one or several compensation coils. With appropriate manual adjustment, the three components of a magnetic field homogeneous in the vicinity of the coils or also its share with a homogeneous gradient can largely be suppressed with such gradiometers and the inhomogeneous biomagnetic near field which is still heavily non-uniform in the vicinity of the gradiometer can be measured. The R-F SQUID is furthermore inductively coupled to a resonant tank circuit, the high frequency voltage of which is phase- or amplitude-modulated by the input signal. Generally, the operating point of the R-F SQUID is fixed by negative feedback via an additional compensation coil and the compensation current is used as a signal which can be evaluated electronically.
The R-F SQUIDs employed in the known systems have a characteristic noise signal. See, for instance, "SQUID Superconducting Quantum Interference Devices and Their Applications", Berlin/New York, 1977, pages 395 to 431. In order to determine the mentioned extremely weak magnetic fields, a signal average must be formed at the individual measuring points with the aid of a multiplicity of individual measurements, for at least the following three reasons, namely, because of the incompleteness of the interference field suppression, because of interference components of biological origin in the near field, and because of the intrinsic noise of the sensor. In order to obtain a three-dimensional field distribution measurements must be taken, in addition, sequentially in time at different locations of the area to be examined. With these measuring methods, the difficulty then exists that coherence of the field data no longer exists over the measuring time required therefor, and in addition, measuring times are obtained which can no longer be demanded clinically.
It has therefore been proposed to take a multi-channel measurement instead of the known single-channel measurement. See, for instance, "Physica", vol. 107 B, 1981, pages 29 and 30. The mutual interference of the channels in an adjacent arrangement as well as the intrinsic noise of the individual channels can be reduced through the use of direct current (d-c) SQUIDs in place of R-F SQUIDs. See, for instance, "IEEE Transactions on Magnetics", vol, MAG-19, no. 3, May 1983, pages 835 to 844. The adjustment of the individual channels of such a multi-channel gradiometer system of modular design is difficult to manage, however.
It is an object of the present invention to provide, for a multichannel device for the measurement of biomagnetic fields, a gradiometer coil system which permits a relatively simple adjustment of all gradiometer coils, which adjustment has to be made only once. The active area of the coil system should be kept as small as possible so that the cryostat which is required and which should be made as small as possible, can be brought as close as possible. It should further be possible to determine with this coil system, the spatial distribution of the magnetic fields during reasonable measuring times, and coherence of the field data should be largely ensured.